1. Field of the Invention
The present invention relates to a recursive noise reduction device, and more particularly, to a recursive noise reduction device used to reduce noise of stationary regions in moving images.
2. Description of the Related Art
A common recursive noise reduction filter is a device that reduces the noise of a digital image while performing a recursive addition computation. A recursive addition computation is known to perform the addition of a current image multiplied by a constant K and the result of the previous recursive addition computation multiplied by (1 - K) . The image of a frame (or the field) that is presently entered is present image An+1 and that the immediate past image, the image of a frame (or the field) before the present image is An. The image of the frame (or the field) before the immediate past image is An-1. At this time, the equation for the result of the previous recursive addition computation Bn is as follows: Bn=(K.times.An)+[(1-K).times.(Bn-1)]. In this equation, Bn-1 is the result of the previous recursive addition computation which was performed before the last one. If K=1 in the results Bn of the previous recursive addition computation, then Bn=An. Consequently, results Bn of the previous recursive addition computation has a correlation with the immediate past image An.
One conventional kind of a recursive noise reduction device is shown, for example, in FIG. 4. In this Figure, image signals input from an image terminal 41 are multiplied by a value K (O.ltoreq.K.ltoreq.1) by using a multiplier 410. The output of the multiplier 410 passes through an adder 412 and is then output from an output terminal 46, as well as being stored in a memory 43.
Moreover, past signals having a correlation with the image signals input from the input terminal 41 are extracted from the memory 43 and are multiplied by a value (1-K) by a multiplier 411, and multiplied signals from multiplier 411 are added by the adder 412 to the output signals of the multiplier 410. The result is output from the output terminal 46 and simultaneously is again stored in the memory 43. Moreover, the value of the coefficient K is fixedly set according to the amount of the noise components.
Generally, in stationary images, because the noise of the same image is uncorrelated between each frame, reduction of the noise components of the image signals is performed by repeating the above-mentioned operation.
The relationship of the input image signals x and the output image signals y is shown by the following equation (Equation 1). In this instance, x' is the output signal of the memory 43. EQU y=Kx+(1-K)x' (Equation 1)
Modifying Equation 1 by setting 1-K=s, it becomes the following equation (Equation 2): EQU y=x-s(x-x') (Equation 2)
FIG. 5 shows a circuit in which Equation 2 is realized. In the circuit of FIG. 5, the second term of Equation 2, i.e., (x-x'), is effected by the adder 510, and by using a Look Up Table (LUT) stored in a ROM 512, s times this is calculated. Furthermore, the result, s(x-x'), is subtracted from the input image signal by adder 511, and is stored in the memory 53. Moreover, this circuit is well known from Japanese Examined Patent Publication 59-17580.
On the other hand, in contrast to such a prior art noise reduction device, the present inventor initially proposed a device which performed a gradual reduction (1/n) of the value of K according to the number n of cyclic calculations, making the amplitude of the noise of the output image signals 1/.sqroot.n, and which is able to perform a more effective reduction of noise (Japanese Patera Application 4-176019).
The prior art noise reduction device is one which sets the value of the coefficient K as a fixed value. In this kind of prior art device, namely when the value of K is large, for one with a fast convergence of the degree of noise reduction, the convergence value does not become small. Vice versa, when the value of K is small, for one whose convergence value of the degree of noise reduction becomes small, the convergence becomes slow in a stage in which the number of times of cyclic additions is small. In the device proposed by the inventor, the value of K is made large, and in addition, for the number of times of cyclic additions becoming large, by gradually making the value of K smaller, a device is realized in which the convergence of the degree of noise reduction is fast and also the convergence value is small.
Nevertheless, in such a device previously proposed by the present inventor, for a region in which standstill is continuing, noise reduction is rapid and its effect is large. However, the problem is that for regions in which there is movement in the image, particularly after the value of K has become small, hardly any effect of noise reduction is obtained for regions in which there has been movement of the image.